The electrical power that runs the equipment used in domestic and business operations is obtained from a complex distribution system that communicates power generated at large production plants across a grid to local generating stations and substations. The generating stations and substations are interconnected with the local businesses and households via a network of utility lines that communicate the power. Distributed electric power generation that convert power from photovoltaic devices, micro-turbines, or fuel cells can function in conjunction with the grid to supplement to power supply from the main plant. Thus, power may be generated from different sources and then combined to feed the power network. Loads that are connected to the grid take the generated power and convert it to a usable form or for supplementing the grid. The control, monitoring, and integration of various electrical power supplies is a complex operation that can sometimes lead to discontinuities and interruptions in the smooth distribution of electrical power to the equipment reliant on this power supply.
The number and types of independent energy sources is growing rapidly, and can include photovoltaic devices, wind, hydro, fuel cells, storage systems such as battery, super-conducting, flywheel and capacitor types, and mechanical means including conventional and variable speed diesel or IC engines, Stirling engines, gas turbines, and micro-turbines. Each of these independent energy sources needs some type of power converter that feeds energy to the grid or used to directly power the various loads. There must also be some means to provide protection when the grid becomes unstable or there is a fault in the system.
A problem with present electrical power distribution systems at the consumer end of the chain is the application the integrated power supply to a unbalanced or non-linear load condition, and the unwanted harmonics that can be generated by non-linear loading conditions. In distributed power applications, high harmonic content or unbalanced loads may lead to inefficiency, resonances, equipment malfunction or damage, and other unanticipated distribution system behavior. This high harmonic feedback can also result in damage to equipment and possibly personal injury. Power conditioners and harmonic “eaters” are devices that can be used to protect equipment from unsteady power supplies.
Power converters, including inverters coupled to a DC source, are used as back-up power supplies to accommodate the lapses or gaps in the power supplied by the distribution system, and are particularly applicable when power is integrated with newer energy generating devices such as photovoltaic devices, micro-turbines, variable speed internal combustion (IC) engines, fuel cells, and superconducting storage. These devices generate AC or DC electricity that needs to be converted to a conditioned AC for feeding the connected loads.
Uninterruptible power supplies (UPS) systems are devices that are commonly used to stabilize and maintain a back-up constant power supply for use in the event of an interruption in the main power distribution system. UPSs are used to compensate for voltage sags in the line voltage and provide instantaneous back-up voltage to equipment when the primary voltage power is interrupted. This can be critical to certain devices that cannot tolerate power interruptions, such as computers, medical devices, and safety equipment. The quality of the power supplied by a UPS system is affected by various factors, including the quality of the output voltage regulation, the total harmonic distortion introduced by the UPS into the power distribution system, the output impedance of the UPS, the response of the UPS to transient events in the line voltage, and the response of the UPS to non-linear or distorted load requirements. Feedback control systems that control the UPS voltage, frequency and amplitude are pivotal to enhance the quality of the UPS output. An example of an arrangement and operation of a UPS and its controls is described in U.S. Pat. No. 6,768,223 to Powell et al., issued Jul. 27, 2004, the contents of which are fully incorporated herein by reference.
Prior art controllers for UPS systems traditionally use a single voltage control loop using proportional-integral (PI) control laws or proportional-integral-derivative (PID) control laws. These controllers may include a pulse width modulated frequency generator to smooth the frequency output to match the requirements of the particular load served. U.S. Pat. No. 5,654,591 to Mabboux et al., issued Aug. 5, 1997, the contents of which are fully incorporated herein by reference, illustrates the use of both of these types of controllers in a UPS system. PI controllers and PID controllers, collectively referred to herein as “classic” controllers, offer the benefits of minimal steady state error and are extremely stable, but classic controllers are ill-equipped to handle harmonic distortion at the output voltage which are exacerbated by non-linear loads. The transient response of a classic controller can also be problematic, with response time on the order of 5–50 milliseconds. Also, there is a typically drop in the voltage of a system using a classic controller when a full load is applied, and this voltage drop is proportional to the impedance of the system.
Another, less frequently used type of controller is the state space controller which is based on the set of “state” variables solved by differential calculus. An example of a state space controller is described in U.S. Pat. No. 5,047,910 to Levran et al., issued Sep. 10, 1991, the contents of which are fully incorporated herein by reference. State space controllers exhibit very good transient response time (less than 1 ms) and very low harmonic distortion in the range of one percent or less. However, several drawbacks exist in the use of state space controllers that largely exclude their use in most applications, including a relatively large steady-state error associated with the use of state space controllers that may be as high as 10% of the full load, an instability that can result in a modulation of the output voltage, and a frequency inconsistency with pulse width modulation that varies with conditions such as load, filter components, and DC bus voltage.
The art is in need of a controller for a feedback system that can eliminate or substantially reduce the steady state error while simultaneously addressing the transient response and harmonic output characteristic of non-linear loads.